Based on the energy coupling and transformation mechanism of giant magnetostrictive materials(GMM), the magnetic field and mechanical field were simulated using the finite element method (FEM) for the GMM actuator. And the convergence abilities under different prestress values and different load types were investigated. Then the calculated deformations were compared with the experimental values. The results convinced that the FEM iteration method was suitable for the simulation of energy coupling and transformation mechanism of the GMM. At last, the property between input currents, prestresses, load types and output deformations were studied. All these shown that there was a good compromise between good linearity and large strain under the prestress 6Mpa.